linux-kernel - Re: [PATCH v7 1/3] x86: Add classes to exception tables

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Message-ID: <20160108172035.GE12132@pd.tnic>
Date:	Fri, 8 Jan 2016 18:20:35 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	Robert <elliott@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v7 1/3] x86: Add classes to exception tables

On Fri, Jan 08, 2016 at 04:29:49PM +0000, Luck, Tony wrote:
> I thought the guideline was that new features are GPL, but changes
> to existing features shouldn't break by adding new GPL requirements.
> 
> The point is moot though because  the shared hallucinations wore
> off this morning and I realized that having the "handler" be a pointer
> to a function can't work. We're storing the 32-bit signed offset from
> the extable to the target address. This is fine if the table and the
> address are close together. But for modules we have an exception
> table wherever vmalloc() loaded the module, and a function back
> in the base kernel.

Whoops, true story.

> So back to your ".long 0" for the default case.  And if we want to allow
> modules to use any of the new handlers, then we can't use
> relative function pointers for them either.
> 
> So I'm looking at making the new field just a simple integer and using
> it to index an array of function pointers (like in v7).

Right, that sounds good too. I guess we can even split the integer into

[0 ... 7][8 ... 31]

where slice [0:7] is an index into the handlers array and the remaining
unused 24-bits could be used for other stuff later. Normal addition as a
way to OR values should work.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.